This section is intended to introduce various aspects of the art, which may be associated with embodiments of the disclosed techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the disclosed techniques. Accordingly, it should be understood that this section is to be read in this light, and not necessarily as admissions of prior art.
Hydrocarbons are generally produced using a series of pipelines to transfer the hydrocarbons from a wellhead to production facilities. The production of hydrocarbons is controlled using pressure and flow rates within the pipelines, which may be referred to as process control. Topside process control is typically accomplished by throttling a gas or liquid stream through a control valve in order to control pressure or flow rates. However, subsea valve technology may not operate using topside control valves due to the harsh environmental conditions that occur subsea. Likewise, pneumatic actuation may not be used in subsea process control due to subsea environmental conditions, specifically, the compressibility of air.
Electric actuation may be used in subsea process control but may not be widely used subsea due to the unproven operation of electrical actuation. As a result, electrical actuation is typically used in actuators which provide only on/off or stepping control functions.
Hydraulically controlled chokes may also be used to throttle flow streams subsea. Choke valves are discretely positioned to predetermined points and travel at relatively slow speeds. As a result, hydraulic controls in choke valves are unable to accommodate changes in a flow stream at the response speeds needed for efficient process control.
Alternatively, subsea pump assisted hydraulic circuits may be used to throttle flow streams. In this scenario, a hydraulic circuit may be supplemented with the use of a subsea pump to boost the flow rate to the valve for open and close functions. However, the pump exhibits a slow response at the start of the valve cycle, approximately for 2%-10% of the valve movement. Further, the pump motor may be extremely stressed during service, and may lack high reliability. As such, the pump has a possibility of increased operation and maintenance requirements. Various examples of techniques avoid such slow valve movements are discussed in the paragraphs to follow.
U.S. Pat. No. 7,237,472 by Cove (hereinafter “Cove”), discloses a linear hydraulic stepping actuator with fast close capabilities. A choke system with hydraulic circuits may provide choke valve positioning that can be varied by the use of incremental steps. The incremental movement action in either the opening or closing direction may be accomplished through the use of one of the two hydraulic slave cylinders. A fast close system may be used which may provide valve control in a fast close line to move the choke actuator to the full closed position from anywhere in the travel over a shorter period of time than through normal stepping operation, instead of running through a series of steps to close the valve. However, even in the presence of a fast close line to move the choke actuator to full closed position, a choke system is unable to accommodate changes in a flow stream at the response speeds necessary for efficient process control.
U.S. Pat. No. 6,729,130 by Lilleland (hereinafter “Lilleland”), discloses a device in a subsea system for controlling a hydraulic actuator and a subsea system with a hydraulic actuator. The hydraulic actuator may be connected to a supply line for supply of a supply fluid to the actuator and a return line for removal of a return fluid from the actuator. However, the supply fluid to the hydraulic actuator may not be enough to ensure the response speeds for efficient process control.